<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .container {
            display: flex;
            width: 1000px;
            height: 200px;
            border: solid 1px red;
        }

        .container .item {
            border: solid 1px;
            height: 200px;
        }

        .container .item1 {
            width: 500px;
        }

        .container .item2 {
            flex-basis: 500px;
        }
    </style>
</head>
<body>
<div class="container">
    <div class="item item1">width</div>
</div>
<div class="container">
    <div class="item item2">flex-basis</div>
</div>
</body>
</html>